Improving Our Understanding of Environmental Controls on the Distribution of C3 and C4 Grasses STEPHANIE PAU*, ERIKA J

Global Change Biology (2013) 19, 184–196, doi: 10.1111/gcb.12037 Improving our understanding of environmental controls on the distribution of C3 and C4 grasses STEPHANIE PAU*, ERIKA J. EDWARDS† andCHRISTOPHER J. STILL‡§ *National Center for Ecological Analysis and Synthesis (NCEAS), 735 State Street, Suite 300, Santa Barbara, CA 93101, USA, †Department of Ecology and Evolutionary Biology, Brown University, Providence, RI 02912, USA, ‡Department of Geography, University of California, Santa Barbara, CA 93106-4060, USA, §Forest Ecosystems and Society, Oregon State University, 321 Richardson Hall, Corvallis, OR 97331-5752, USA Abstract A number of studies have demonstrated the ecological sorting of C3 and C4 grasses along temperature and moisture gradients. However, previous studies of C3 and C4 grass biogeography have often inadvertently compared species in different and relatively unrelated lineages, which are associated with different environmental settings and distinct adaptive traits. Such confounded comparisons of C3 and C4 grasses may bias our understanding of ecological sorting imposed strictly by photosynthetic pathway. Here, we used MaxEnt species distribution modeling in combination with satellite data to understand the functional diversity of C3 and C4 grasses by comparing both large clades and closely related sister taxa. Similar to previous work, we found that C4 grasses showed a preference for regions with higher temperatures and lower precipitation compared with grasses using the C3 pathway. However, air temperature differences were smaller (2 °C vs. 4 °C) and precipitation and % tree cover differences were larger (1783 mm vs. 755 mm, 21.3% vs. 7.7%, respectively) when comparing C3 and C4 grasses within the same clade vs. comparing all C4 and all C3 grasses (i.e., ignoring phylogenetic structure). These results were due to important differences in the environmental preferences of C3 BEP and PACMAD clades (the two main grass clades). Winter precipitation was found to be more important for understanding the distribution and environmental niche of C3 PACMADs in comparison with both C3 BEPs and C4 taxa, for which temperature was much more important. Results comparing closely related – C3 C4 sister taxa supported the patterns derived from our modeling of the larger clade groupings. Our findings, which are novel in comparing the distribution and niches of clades, demonstrate that the evolutionary history of taxa is important for understanding the functional diversity of C3 and C4 grasses, and should have implications for how grasslands will respond to global change. Keywords: BEP, Echinochloa, land surface temperature, niche conservatism, Oplismenus, PACMAD, temperature crossover model Received 27 April 2012 and accepted 5 September 2012 temperature controls on C -C distributions is the bio- Introduction 3 4 chemically based temperature crossover model (Ehle- C3 and C4 grasses are two fundamental plant functional ringer et al., 1997), which predicts that C4 plants will be types (PFTs) that play important and distinct roles more competitive and thus more abundant in grassland in ecosystem functions such as global terrestrial regions where the mean monthly air temperature ° productivity and water cycling. Although C4 plants exceeds 22 C (Collatz et al., 1998; Still et al., 2003a). account for 20–25% of global terrestrial productivity Although the temperature crossover model is a power- (Still et al., 2003a), large uncertainties remain regarding ful approach for predicting regions and climates where their response to climate variability and future global C4 grasses should be dominant over C3 grasses, it has change. Part of this uncertainty stems from our ability several drawbacks. For example, certain regions where to characterize the environmental, ecological, and evo- C4 grasses are known to be dominant, such as the lutionary controls on C3 and C4 grass distributions. cooler high plateaus of South Africa, are not captured Numerous studies have demonstrated the ecological by this approach because they do not experience mean ° sorting of C3 and C4 grasses along spatial gradients, monthly temperatures in excess of 22 C (O’Connor & particularly temperature gradients (reviewed in Sage & Bredenkamp, 1997). Another limitation of the crossover Monson, 1999). The prevailing explanation for temperature model is that C3 and C4 grasses are each assigned separate but uniform temperature thresholds; Correspondence: Stephanie Pau, tel. 00+1 805 892 2500, fax 00+1 thus, the large functional and ecological diversity that 805 892 2510, e-mail: [email protected] exists in grasses is not captured. 184 © 2012 Blackwell Publishing Ltd THE DISTRIBUTION OF C3 AND C4 GRASSES 185 There is long-standing observational evidence of preferences of C3 and C4 grasses within and among C3-C4 partitioning along moisture gradients (Chazdon, evolutionary lineages, we can refine our understanding 1978; Vogel et al., 1986). Precipitation amount and of differences due principally to photosynthetic drought tolerance should be important in C3-C4 pathway as well as highlight functional diversity across distributions because physiological and biochemical lineages. Phylogenetically structured comparisons differences related to photosynthetic pathway produce should improve our ability to predict the distribution, higher rates of carbon uptake per unit of transpired abundance, and ecological success of C4 photosynthesis water (i.e., photosynthetic water-use efficiency) in C4 and associated ecosystem functions in response to grasses (Pearcy & Ehleringer, 1984). More importantly, future climate change. the interaction between temperature and precipitation Here, we investigate the ecological setting and envi- should be a key in understanding the distribution of ronmental niche of C3 and C4 grasses in a phylogenetic these PFTs. Both seasonal and interannual variation in context – comparing clades and sister taxa – to provide temperature and precipitation have been shown to be novel information on the distribution and functional strongly related to C3 and C4 grass distributions, with aspects of these grasses. To explore how a variety of – – C3 grasses typically active during the cool-wet season environmental controls beyond air temperature and C4 grasses active during the warm-dry season define C3 and C4 niches and distributions, we utilized a (Teeri & Stowe, 1976; Paruelo & Lauenroth, 1996; species distribution model (SDM) approach (this is also Tieszen et al., 1997; Davidson & Csillag, 2003; Winslow known as ecological niche modeling – see Franklin, et al., 2003; Still et al., 2003b; von Fischer et al., 2008). 2009). SDMs utilize environmental information gath- Despite well-established patterns along temperature ered from geo-referenced collection localities to under- and moisture gradients, research on C3 and C4 grass stand the environmental correlates of species as an ecology can benefit from recent advances in compara- indication of their ecological requirements and to tive methods and in the development of well-resolved model their geographic distributions (Franklin, 2009). grass phylogenies (Edwards et al., 2007; Edwards & We used this approach to quantify and assess the Still, 2008; Edwards & Smith, 2010). Previous studies of abiotic factors that control C3 and C4 distributions (the C3 and C4 ecology have often inadvertently compared fundamental niche) based on species’ localities that species in different and relatively unrelated lineages. incorporate biotic limiting factors (the realized niche) Most C3 grasses are found in two distinct lineages (see Franklin, 2009 for a discussion of niche concept in whose last common ancestor likely lived between 50 SDMs). Species distribution modeling is an increasingly and 80 Mya (Vicentini et al., 2008). Likewise, there are popular methodology applied to problems in conserva- – an estimated 22 24 independent origins of the C4 path- tion biology, biogeography, ecology, and systematics way (Grass Phylogeny Working Group II, 2012). Conse- (reviewed in Guisan & Thuiller, 2005). We extend the quently, there should be considerable diversity within SDM approach to better understand the ecological and C3 and C4 groups associated with different lineages, functional diversity of higher levels of taxonomic distinct environmental settings, and associated adap- organization. Using niche modeling within a phyloge- tive traits that confound our understanding of netic context to examine major PFTs and large clades ecological differences imposed strictly by photosyn- (as opposed to individual species) is a novel application thetic pathway (Edwards & Still, 2008; Edwards & of this modeling approach (Huntley et al., 2004; Guisan Smith, 2010; Taylor et al., 2010). In other words, photo- & Thuiller, 2005; Chapman & Purse, 2011; see Methods synthetic pathway may be associated with other traits and Discussion for further discussion of modeling the conserved through evolutionary history. For example, aggregated niches of clades). recent work has suggested that most grasses are We also exploited several satellite-derived datasets to warm-climate specialists, regardless of photosynthetic characterize the environmental niches of these grasses; pathway, and a closer look at the environmental con- such datasets should provide several advantages over text associated with C4 origins indicates that differences station-based air temperature and moisture climatolo- in habitat aridity and irradiance may have played a lar- gies. This may be especially true

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